The present invention relates to optical scanning devices and, more particularly, to an arrangement for scanning the image of a document along a plurality of scan lines and providing electrical signals related to the document image density along the scan lines. Such document scanners find particular application in duplicators or copiers in which printing occurs along print lines on a sheet of copy paper which correspond to the scan lines on the original document. An ink jet printer may advantageously be used in such a copier for depositing drops of ink along the respective print lines on the copy paper.
One such copier is shown in U.S. Pat. No. 4,106,061, issued Aug. 8, 1978, to Burnett. In the Burnett copier, the tone of a document image at successive points along parallel scan lines is sensed by means of a photodiode array, with each diode in the array receiving light reflected from points along an associated one of the scan lines on the document. Scanning is accomplished by means of a rotating mirror which is rotated in synchronism with the movement of a sheet of copy paper on a paper supporting drum. An ink jet print head mounted adjacent to the drum provides a plurality of jets, with each jet corresponding to one of the light-sensitive diodes. The outputs from the diodes are supplied for processing to a circuit which effectively combines gray scale and line copy processing techniques.
In general, when a document is scanned in a line copy mode, the measured image density is compared to a selected threshold level and if the density exceeds this threshold level, a print control pulse is supplied to the ink jet print head to cause a drop from the associated jet to be deposited on the print line. Thus, the resulting image has portions which are covered completely with ink and other portions which are left white. In a gray scale printing mode, however, drops are deposited in varying densities in areas of the copy paper to produce, collectively, the appearance of varying levels of gray. Thus, gray scale reproduction may be used to print an acceptable copy of a photograph, whereas a copy of the photograph which would result from line copy printing would be totally unacceptable. Various gray scale processing techniques, such as shown in the Burnett patent, have utilized integrator circuits for integrating the outputs from each of the photodiodes. When the integrated outputs exceed a predetermined threshold value, a print control pulse is generated. Burnett combines the line copy and gray scale circuits such that scanning across a thin black mark on the document results in immediate generation of a print control pulse, regardless of the operation of the integrator.
Various other gray scale techniques have been developed which provide for appropriately spaced dots or other marks within large areas on the copy such that the gray scale effect is achieved. Such systems are shown in U.S. Pat. No. 4,004,079, issued Jan. 18, 1977, to Boston; U.S. Pat. No. 4,084,259, issued Apr. 11, 1978, to Cahill et al; U.S. Pat. No. 4,080,634, issued Mar. 21, 1978, to Schreiber; and U.S. Pat. No. 3,604,846, issued Sept. 14, 1971, to Behane et al.
U.S. Pat. No. 4,122,496, issued Oct. 24, 1978, to Childress et al discloses a scanning arrangement in which scan signals are developed in correspondence to the light reflected from successive concentric sharp and unsharp areas of the document being scanned. The sharp and unsharp data signals are combined in accordance with a sharpness factor, having a predetermined value in accordance with the original image being scanned, to produce a succession of digital gray level data signals.
One rather recently developed scanning technique utilizes a charge coupled device (CCD) which defines an array of photosensitive sites which accumulate charges in dependence upon an image projected thereon. Such CCD imaging arrays are shown in U.S. Pat. Nos. 4,032,976, issued June 28, 1977, to Levine, and U.S. Pat. No. 4,117,514, issued Sept. 26, 1978, to Terui et al. In the Terui et al patent, a scanner is disclosed having vertical columns and horizontal rows of photosensors which transfer charges into vertical shift registers. The vertical shift registers shift the charges sequentially into corresponding stages in a horizontal shift register. The horizontal shift register is shifted rapidly to produce lines of scan data in a raster scan format. The Levine patent discloses a similar scanner arrangement, further including a temporary storage CCD array which is positioned between the photosensor array and the horizontal shift register.
U.S. Pat. No. 3,833,762, issued Sept. 3, 1974, discloses a CCD imager array in which charges accumulated within charge collecting regions of charge transfer channels are shifted in synchronization with the movement of an image projected upon the array. An image motion detector determines the rate of image motion and provides for charge transfer at a corresponding rate. This permits substantial imaging time for scanning a moving image without smearing the image. This technique presents a significant advantage over prior techniques which would require sampling at a relatively high rate to reduce image smear.
As discussed in "Time Delay and Integration Charge Coupled Devices (CCDs) Applied to the Thematic Mapper," Leslie L. Thompson, SPIE, Vol. 143 (1978), pp. 19-26, and "A High Resolution Solid Charge Coupled Device (CCD) Space-borne Earth Imager," Irving Hirschberg, SPIE, Vol. 143, (1978), pp. 2-10, shifting of charges in a CCD imager array in synchronization with image motion results not only in elimination of image smearing, as proposed by the Gudmunsen patent, but also provides the advantage that each pixel of the image being scanned is scanned by a succession of photosensors. The effective scan time for each pixel is therefore increased, thus permitting scanning at lower light levels and, additionally, increasing the signal to noise ratio and averaging out variations in photosensor response characteristics.
A need exists for a reliable image scanner for use in a copier including an ink jet printer which is capable of operating at high scanning rates with minimal document illumination and, additionally, which provides for the generation of print control data pulses in either a line copy mode or a gray scale mode.